Methods of fabricating devices by low pressure cold welding

ABSTRACT

Methods of transferring a metal and/or organic layer from a patterned stamp, preferably a soft, elastomeric stamp, to a substrate are provided. The patterned metal or organic layer may be used for example, in a wide range of electronic devices. The present methods are particularly suitable for nanoscale patterning of organic electronic components.

[0001] This application claims the benefit of U.S. ProvisionalApplication No. 60/435,350, filed Dec. 20, 2002, which is incorporatedherein by reference in its entirety. The subject matter of thisapplication is related to U.S. Pat. No. 6,468,819, U.S. Pat. No.6,407,408, concurrently pending patent application Ser. Nos. 09/802,977,09/833,695, and 09/899,850, all of which are incorporated herein byreference in their entireties.

FIELD OF THE INVENTION

[0002] The present invention relates to a method of fabricating adevice, and more particularly to the transfer of a metal or organiclayer from a patterned stamp to a substrate.

BACKGROUND OF THE INVENTION

[0003] Nearly all electronic and optical devices require patterning.Patterned metals are used in forming a variety of such devices. Forexample, patterned metals may be used in forming transistors, aselectrodes in various devices, and as shadow masks in the patterning ofvarious materials. One possible use for patterned metals is aselectrodes in organic light emitting devices (OLEDs), which make use ofthin films that emit light when excited by electric current. PopularOLED configurations include double heterostructure, singleheterostructure, and single layer, and may be stacked, as described inU.S. Pat. No. 5,707,745, which is incorporated herein by reference inits entirety.

[0004] Patterning of sub-micrometer structures is preferable for therealization of new and improved types of devices such as flat paneldisplays.

[0005] For OLEDs from which the light emission is only out of the bottomof the device, that is, only through the substrate side of the device, atransparent anode material such as indium tin oxide (ITO) may be used asthe bottom electrode. Since the top electrode of such a device does notneed to be transparent, such a top electrode, which is typically acathode, may be comprised of a thick and reflective metal layer having ahigh electrical conductivity. In contrast, for transparent ortop-emitting OLEDs, a transparent cathode such as disclosed in U.S. Pat.Nos. 5,703,436 and 5,707,745 may be used. As distinct from a transparentor bottom-emitting OLED, a top-emitting OLED is one which may have anopaque and/or reflective substrate, such that light is produced only outof the top of the device and not through the substrate, or can be afully transparent OLED that may emit from both the top and the bottom.

[0006] As used herein, the term “organic material” includes polymers aswell as small molecule materials that may be used to fabricate OLEDs.The organic materials of an OLED are very sensitive, and may be damagedby conventional semiconductor processing. For example, any exposure tohigh temperature or chemical processing may damage the organic layersand adversely affect device reliability.

SUMMARY OF THE INVENTION

[0007] An embodiment of the invention is directed to a method offabricating a device by depositing a metal layer over a patterned stamp,and then transferring the metal layer from the patterned stamp onto asubstrate. Preferably, the patterned stamp is a soft, elastomeric stamp.

[0008] An embodiment of the invention is also directed to a method offabricating a device by depositing one or more organic layers over apatterned stamp, and then transferring the organic layer(s) from thepatterned stamp onto a substrate. Preferably, the patterned stamp is asoft, elastomeric stamp. A combination of metal and organic layers mayalso be transferred from a patterned stamp onto a substrate.

[0009] An embodiment of the invention includes transferring a depositedmetal layer from a patterned stamp onto a substrate by cold-welding.According to this embodiment, a strike layer is optionally depositedover a substrate and a patterned stamp is obtained having a metal layerdeposited over the stamp. The stamp is then pressed onto the substrate,such that the metal layer over the patterned stamp contacts portions ofthe strike layer or other underlying layer, and sufficient pressure isapplied to cold-weld the metal layer to the strike layer or otherunderlying layer. The patterned stamp is removed and the portions of themetal layer that are cold-welded to the strike layer or the substratebreak away from the stamp and remain cold-welded to the strike layer orthe substrate, in substantially the same pattern as the patterned stamp.

[0010] Another embodiment of the invention includes transferring adeposited organic layer from a patterned stamp onto a substrate by“cold-welding”. According to this embodiment, a strike layer isoptionally deposited over a substrate and a patterned stamp is obtainedhaving an organic layer deposited over the stamp. The stamp is thenpressed onto the substrate, such that the organic layer over thepatterned stamp contacts portions of the strike layer or the substrate,and sufficient pressure is applied to “cold-weld” the organic layer tothe strike layer or the substrate. The patterned stamp is removed andthe portions of the organic layer that are “cold-welded” to the strikelayer or the substrate break away from the stamp and remain adhered tothe strike layer or the substrate, in substantially the same pattern asthe patterned stamp.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011]FIG. 1 is a schematic illustration of the patterned metal transferprocess according to an embodiment of the present invention.

[0012]FIG. 2 shows the cathode patterning of organic electronic devicesby low pressure cold welding.

[0013]FIG. 3 shows scanning electron microscope (SEM) images of a stampbefore (a, b) and after (c) the metal pattern is transferred.

[0014]FIG. 4 depicts an example of a roll-to-roll process for thefabrication of organic electronic integrated circuits, into whichprocess the present methods may be incorporated.

[0015]FIG. 5 depicts the formation of a PDMS stamp and a methodaccording to an embodiment of the present invention of using that stampto transfer metal to a substrate.

[0016]FIG. 6(a) shows a top view of a master, and FIGS. 6(b) and 6(c)show SEM images of the PDMS stamp fabricated from the master shown inFIG. 6(a).

[0017]FIG. 7 shows a cross-section of a patterned stamp adapted for usewith an embodiment of the present invention and a substrate, wherein apatterned metal layer will be transferred from the stamp to thesubstrate by cold-welding metal over the stamp to portions of a thinmetal film over the substrate.

[0018]FIG. 8 shows a cross-section of the stamp and the substrate ofFIG. 7 after portions of the metal from the stamp have been transferredto the substrate in accordance with an embodiment of the method of thepresent invention.

[0019]FIG. 9 shows a cross-section of the substrate of FIG. 8 having thepatterned metal thereover, before (FIG. 9(a)) and after (FIG. 9(b))portions of the thin metal film are removed in accordance with oneembodiment of the present invention.

[0020]FIG. 10 shows a cross-section of the substrate of FIG. 9(b) havingthe patterned metal thereover, before (FIG. 10(a)) and after (FIG.10(b)) portions of the substrate are etched to form a patternedsubstrate having essentially the same pattern as the stamp in accordancewith another embodiment of the present invention.

[0021]FIG. 11 shows a cross-section of the substrate of FIG. 10(b)having the patterned metal thereover, before (FIG. 11(a)) and after(FIG. 11(b)) the remaining thin metal film and patterned metal layer areremoved from the patterned substrate.

[0022]FIG. 12 shows a cross-section of a patterned stamp adapted for usewith an embodiment of the present invention having anadhesion-diminishing layer between the stamp and the metal layerthereover, and a substrate over which a metal material is to bepatterned by cold-welding the metal over the stamp to the thin metalfilm over the substrate, in which an organic layer is positioned betweenthe substrate and the thin metal film.

[0023]FIG. 13 shows optical microscope images of cathodes fabricated inaccordance with embodiments of the invention.

[0024]FIG. 14 shows the current density (J) vs. voltage (V)characteristics of a 200-μm-diameter organic light-emitting device(OLED) patterned by cold welding (solid circles).

[0025]FIG. 15 shows a preferred shape for the stamp of an embodiment ofthe present invention to avoid side wall deposition of metal on thestamp.

[0026]FIG. 16 shows an embodiment of a hybrid stamp adapted for use withan embodiment of the present invention.

DETAILED DESCRIPTION

[0027] The present invention will be described with reference to theillustrative embodiments in the following processes and drawing figures.

[0028] Methods are provided for forming a patterned metal and/or organiclayer over a substrate using a stamp. Such a patterned metal or organiclayer may be used for example, in forming an electronic device, eitheras part of the device itself or as a mask in patterning other layers ofthe device or the substrate.

[0029] In one embodiment of the invention, a method is provided forforming a patterned metal layer over a substrate using a stamp. Thisembodiment may be used to produce features of submicron scale based onthe transfer of a metal layer from a stamp to a substrate assisted bythe atomic scale process of metallic cold welding. The patterned metallayer can be used for example, as an etch mask to replicate the patternon the substrate, or the layer itself can serve as contact electrodesfor a wide range of electronic devices. Given the very high patternresolution and its compatibility with organic electronics, thistechnique holds promise for application to the formation of contacts todevices at the single molecule scale.

[0030] Cold welding is the formation of a metallic bond at about roomtemperature between two metal surfaces by application of pressure.Preferably, the two metal surfaces are of like composition. Cold weldingis used to bond separate metal parts of macroscopic size. The surfacesbond to each other when the interfacial separation is decreased below acritical value, resulting in a single solid. In order to achieve goodpatterns by this technique, the applied pressure should be high enoughto decrease the interfacial separation below the critical value. Thatis, the applied pressure should be high enough to decrease theinterfacial separation of the thin metal film and metal layers below thecritical value. The present methods preferably use cold welding fortransferring any desired metal layer from a patterned, soft elastomericstamp to a substrate; although other bonding methods known to those inthe art are contemplated and within the scope of the invention.

[0031] In another embodiment of the invention, a method is provided forforming a patterned organic layer over a substrate using a stamp.Although not wishing to be bound by any particular theory, it isbelieved that in this embodiment of the invention, organic-to-organicvan der Waals forces are the mechanism for bonding one organic layer toanother organic layer. That is, in this embodiment, instead of a metallayer in contact with a metal layer, an organic layer is in contact withan organic layer with sufficient pressure such that the two organiclayers are bound together. For example, van der Waals forces may beresponsible for such bonding. The term “cold-welding” is used herein toalso refer to this organic-to-organic bonding, even though the term“welding” is typically used only in connection with metal-to-metalbonding.

[0032] As the transferred organic layer may comprise an active componentof a device, this embodiment of the method of the invention can be usedto fabricate, for example, organic integrated circuits such as organicactive matrix displays. When incorporated into roll-to-roll processes,these methods are very suitable for high-throughput and cost-effectivefabrication of organic electronic devices. Although many of theembodiments described herein include the transferring of a metal layer,it should be understood that the metal layers therein could be replacedby organic layers, and thus the metal-layer-transferring embodiments ofthe invention should also be viewed as illustrative of possibleorganic-layer-transferring embodiments of the invention.

[0033] The methods of the present invention are particularly suitablefor nanoscale patterning of organic electronic components, where the wetprocesses used in conventional photolithography might damage underlyingorganic materials.

[0034] Patterning techniques based on stamps or molds provide practicaladvantages, and they are in principle free from the limitations ofoptical diffraction. The present cold welding technique has theadvantages of both simplicity and high resolution common to othermethods based on stamping or molding. In contrast to other techniques,however, it is well suited for direct patterning of organic electronic,or at the highest pattern resolutions, molecular electronic devices. Forexample, previously described techniques use polymer films heated totemperatures higher than their glass transition temperatures, whileothers involve wet chemicals, both features being incompatible with manyfragile molecular solids.

[0035] U.S. Pat. No. 6,468,819, which is incorporated herein byreference, describes direct patterning of organic electronic devicesusing cold welding followed by lift-off of cathode metals on selectedregions of an organic semiconductor thin film. Thus, a patterned metalfilm deposited onto the surface of the organic layers was obtained by asubtractive process, whereby metal on unwanted areas was removed whenthe stamp was separated from the substrate.

[0036] Furthermore, the present additive methods allow for thefabrication of such organic electronic devices as organic thin filmtransistors (OTFTs) where damage inflicted by application of excessivepressure to the active device regions should be avoided. Indeed, therange of applications of the additive process is not confined to organicelectronics: this method may also find uses wherever ultrahighresolution metal patterns are required, such as in adding low resistancemetal bus lines to passive matrix displays and memories. As a result,the methods of the invention are well suited for roll-to-roll processingof organic integrated circuits where contacts for various componentssuch as OTFTs, organic light-emitting diodes (OLEDs), solar cells, andphotodetectors must be simultaneously patterned.

[0037] In an embodiment of the invention, prior to transferring themetal layer, a metal layer is deposited over a patterned stamp having atleast one raised portion such that the metal layer is deposited at leastover the raised portions of the stamp. The stamp is patterned in that ithas raised and depressed portions that form a desired pattern. The stampmay be patterned by any method known in the art, such as by lithographyand reactive ion etching. The stamp preferably has sharp edges to avoidside wall deposition of metal on the stamp. Additionally, the shape ofthe stamp may help avoid side wall deposition of a metal on the stamp. Apreferred shape for the stamp is depicted in FIG. 15.

[0038] Preferably, the stamp is made of a substance that is readilypatterned or easily fabricated from a mold. Examples of suitablematerials that may be used to form stamps in accordance with embodimentsof the present invention include soft substances such aspoly(dimethylsiloxane) (“PDMS”), hard substances such as silicon, glass,quartz, steel and hard metals, as well as other materials known to thoseskilled in the art, and combinations thereof.

[0039] In a particularly preferred embodiment of the present invention,the stamp is compliant and is made of a soft, elastomeric material. Anon-limiting example of a suitable soft, elastomeric material for thestamp is PDMS. In addition, other soft, elastomeric materials known tothose of skill in the art may be used to form stamps in accordance withthe present invention. Examples of such suitable materials includepolyurethane and optical adhesives, such as those available from NorlandProducts Inc., of Cranbury, N.J. A representative example of a NorlandOptical Adhesive (NOA) is NOA 73. As used herein, the term “soft,” whendescribing a stamp or its material, is a relative term which denotes astamp or material that can more easily deform around substrate features(including particles) than can a rigid stamp. Thus, the softness of astamp is dependent upon substrate features. For example, for most PDMS,a suitable material for a soft stamp, the value of E (Young's modulus)is within the range of about 0.1 to about 10 MPa, and the value of G(shear modulus) is less than or equal to about 1 MPa. On the other hand,for silicon, a suitable material for a rigid stamp, the value of E(Young's modulus) is equal to about 130 GPa, and the value of G (shearmodulus) is equal to about 30 GPa. These values for E and G for a softand rigid stamp are only representative values, and they do notestablish nor limit suitable ranges for values of E and G for a soft andrigid stamp.

[0040] When a soft, elastomeric stamp is used in embodiments of themethod of the present invention, the force to be applied across thestamp can be more easily applied uniformly such that a lower appliedforce is needed to form a cold-welded bond, as compared to those methodsemploying a rigid stamp. Furthermore, a soft, elastomeric stamp can moreeasily deform around substrate features (including particles) than can arigid stamp. Thus, patterned metal or organic transfer can be achievedat much lower pressures when using a soft, elastomeric stamp, allowingfor the application of pressure directly over a device active regionwith far less likelihood of introducing damage thereto. In addition, theapplied force needed to form a cold-welded bond depends on the roughnessof the surfaces to be bonded, with rougher surfaces generally requiringa higher applied force.

[0041] Additionally, the stamp may be a hybrid stamp, as is shown inFIG. 16. In the embodiment of a hybrid stamp as shown in FIG. 16, thepattern shape is formed in an outer layer 161 of a stiffer material,while a softer inner layer 162 provides conformability.

[0042] Patterning may be achieved by methods known in the art based onthe composition of the stamp. For example, PDMS stamps can be fabricatedusing a method used in ‘soft lithography,’ as described in Y. Xia etal., Unconventional methods for fabricating and patterningnanostructures, Chem. Rev. 99, 1823-1848 (1999). FIG. 5 depicts theformation of a PDMS stamp using a silicon master, as described by Xia etal. After the stamp is formed, the metal to be transferred is thendeposited over the molded stamp. One example of a suitable patterningtechnique when the stamp is made of silicon is lithography using a phasemask and reactive ion etching.

[0043] In a further embodiment of the method of the present invention,prior to transferring the metal layer onto the substrate, a strike layeris deposited onto the substrate, such that the metal layer from thestamp is transferred onto the strike layer. The strike layer can bedeposited using deposition techniques which are known in the art. Thestrike layer serves to facilitate the transfer of the metal layer fromthe stamp to the substrate, and helps to provide a good electricalcontact for the transferred metal layer. Preferably, the strike layer isdeposited as a blanket layer such that patterning is provided by thetransferred metal layer. Examples of suitable materials which may beused as the strike layer include, but are not limited to, metals andorganic materials. For example, when using an embodiment of the methodof the present invention to fabricate an OLED, the strike layer caninclude a layer of Al and LiF, adjacent to a layer of Au.

[0044] The metal layer over the stamp preferably includes a metal thatis capable of cold-welding to a strike layer, such as a thin metal film,over the substrate upon compression of the metal layer to the thin metalfilm.

[0045] Substrates in accordance with embodiments of the presentinvention may be made of any suitable material, including for example,glass, polymers, silicon and plexiglass. The substrate may be rigid,non-rigid, flexible, opaque or transparent. Of the materials that arepresently commercially available, preferred flexible substrates includepolyethylene terephthalate (PET), poly-ethersulphone (PES),polycarbonate (PC), polyethylenenaphthalate (PEN) and polyimide (PI).Each of these materials has advantages and disadvantages that are morefully described in Weaver et al., “Flexible Organic LED Displays,” 2001Soc. Vac. Coaters 505/856-7188, 44th Annual Technical Conf. Proc. (2001)ISSN 0737-5921 (“Weaver et al.”), which is incorporated by reference inits entirety. It is expected that chemical companies will develop newmaterials that are better suited for use as a flexible substrate for thefabrication of OLED displays. It is also expected that variousembodiments of the present invention may be practiced with suchsubstrates when they become available.

[0046] Suitable metals for use as the metal layer and the strike layerpreferably include those known to those skilled in the art as beingcapable of cold-welding to one another. Preferably, the metal layer andthe strike layer are made of non-reactive metals, such as silver andgold that do not form an oxide layer, or the method of the invention iscarried out in vacuum to prevent the formation of an oxide layer. Themetal layer and the strike layer may be made of the same metal ordifferent metals and preferably form a strong cold-welded bond with oneanother when pressure is applied. For example, if the metal layer ismade of gold, the strike layer may be made of gold or silver; and if themetal layer is made of silver, the strike layer may be made of gold orsilver. Other combinations may also be used.

[0047] According to an embodiment of the present invention, in FIG.1(a), a patterned stamp 1 coated with a metal layer 2 is pressed onto asubstrate 4 pre-coated with a very thin (˜10 nm) metal strike layer 3.Cold welding between the two contacting metal layers then occurs whenenough pressure is applied to overcome the potential barrier existing atthe surfaces due to oxidation or surface contamination. Upon separationof the stamp 1 from the substrate 4, the metal layer 2 of the contactedarea on the stamp is transferred to the substrate 4 (FIG. 1(b)). Next,the metal strike layer 3 on the substrate is anisotropically etched toremove the residual strike layer, thereby exposing the substratematerials in regions 7 between the stamped pattern (FIG. 1(c)). Usingthe metal layer left behind as an etch mask, the pattern can be furthertransferred to the substrate 4 (FIG. 1(d)) by various etching methods,for example reactive ion etching (RIE). In addition, the patterned metallayer 2 in FIG. 1(c) can be used as electrodes if the substrate containspre-deposited semiconductor layers necessary to form the desiredelectronic devices or as bus lines.

[0048] According to another embodiment of the invention, more than onemetal layer may be coated on the patterned stamp and then transferred tothe substrate via cold-welding. For example, more than one metal layermay be transferred to the substrate to form a compound cathode, such asa Mg:Ag/ITO compound cathode.

[0049] The embodiment of the invention depicted in FIG. 2 shows cathodepatterning of organic electronic devices by low pressure cold welding.In FIG. 2(a), the elastomeric stamp 101 made of poly(dimethylsiloxane)(PDMS) precoated with a thick metal (Au) layer 102 is pressed onto thesubstrate 104 coated with the organic heterostructure of organic layers111 and the cathode 103, which also acts as a thin metal strike layer103. In FIG. 2(b), upon separation of the stamp 101 from the substrate104, the metal film 102 on the stamp 101 cold-welded with that on thesubstrate 104 remains on the substrate 104. In FIG. 2(c), the strikelayer 103 is removed by Ar sputter etching to electrically isolate thecontacts.

[0050]FIG. 3 shows scanning electron microscope (SEM) images of a stampboth before (FIGS. 3(a) and (b)) and after (FIG. 3(c)) the metal patternis transferred in accordance with an embodiment of the method of theinvention. The stamp has a pattern of an array of 200-μm-diameter posts,as seen in FIG. 3(a). The grooves 31 on the sidewall and rounded edge(FIG. 3(b)) are due to the photoresist mold. During the metal transfer,the Au film is irregularly transferred from the rounded stamp edge (FIG.3(c)), limiting the edge resolution (as seen in FIG. 13).

[0051]FIG. 4 depicts an example of a roll-to-roll process for thefabrication of organic electronic integrated circuits, into whichprocess the present methods may be incorporated. A sheet of plasticsubstrate 41 is rolled out, and translated by cylindrical drums 42, 43.The upper drum 42 has a desired electrode pattern on its surface and iscoated with an adhesion-reduction layer. First, organic layers aredeposited, for example, by organic vapor phase deposition (OVPD) ontothe substrate 41, followed by deposition of the thin metal strike layer45. As the substrate 41 is pressed between the drums 42, 43, the metalfilm on the drum is transferred to the substrate 41 by cold welding.Next, the electrode pattern on the substrate is obtained by a briefmetal etch 46 to remove the strike layer 45. Metal deposition on oneside of the drum allows for a continuous process.

[0052] Electrodes deposited by embodiments of the methods of theinvention may include any material known to those skilled in the art.Preferably, the electrodes are substantially transparent, that is, theyare made of the appropriate materials for achieving transparency, andare fabricated to the appropriate thickness for achieving transparency.Electrodes of embodiments of the present invention are preferablyfabricated from a conductive metal oxide. Preferred materials ofelectrodes according to the present invention, such as transparentelectrodes, include for example indium tin oxide (ITO), MgAg, andaluminum. Preferred non-transparent electrode materials include LiF:Al.Conductive polymers such as polyaniline and poly(3,4-ethylenedioxythiophene)/poly (styrenesulphonate) (PEDOT/PSS) mayalso be used in accordance with the invention.

[0053] In addition, bus lines may also be deposited by embodiments ofmethods of the invention. These bus lines may be made of any suitablemetal or other electrically conductive material, such as gold, silver,aluminum or copper, or any suitable alloy.

[0054] To avoid incomplete pattern transfer, adhesion between the stampand the metal layer on it should be weaker than between any otherinterfaces present in the substrate, and compared to the fracturestrength of the materials employed. Therefore, in a preferred embodimentof the invention, an adhesion-reduction layer 5 is inserted between thestamp 1 and the metal layer 2, as shown in FIG. 1(a). Likewise, whentransferring an organic layer, it is preferable to insert anadhesion-reduction layer between the stamp and the organic layer inorder to avoid incomplete pattern transfer. In cases where thetransferred metal serves as a mask for further etching and patternreplication at the substrate, the metal on the stamp is preferablythicker than that on the substrate for good thickness contrast as shownin FIG. 1(b).

[0055] The metal additive process of an embodiment of the presentinvention differs from the prior subtractive technique in that it reliesprimarily on the cold welding process, while the subtractive methodfurther requires fracture of the metal films. This difference gives thenew additive technique several advantages described below, which make itpractical for use in a roll-to-roll process for continuous patterning oforganic electronic circuits, as depicted in FIG. 4.

[0056] A first advantage is that the present methods use lower pressuresthan prior methods. Cold welding may be achieved for example, between Aufilms at a remarkably lower pressure (about 180 kPa) using a soft,elastomeric stamp (see example 1 herein). That is, when a PDMS stamp isused for example, the pressure required for the additive patterningprocess is approximately 1000 times lower than what has been previouslyreported using a rigid stamp (see C. Kim et al., Science (2000), 288,831; C. Kim et al., Appl. Phys. Lett. (2002), 80, 4051). Thus, this lowpressure cold welding method may be particularly suitable forapplications in which the applied pressure needs to be minimized.Embodiments of the invention in which soft, elastomeric stamps are usedmay optionally be applied to curved substrates and/or stamps.

[0057] However, for the subtractive process, a soft, elastomeric stampis not desirable since the higher pressure may be needed to plasticallydeform the metal film at the pattern edges prior to its selectiveremoval. With a soft, elastomeric stamp, this technique can also beapplied to the fabrication of such vertical geometry devices as OLEDsand solar cells, where the active device areas are placed under directpressure.

[0058] As shown in FIG. 5, when the PDMS stamp 51 is contacted with thesubstrate 54, the metal 52 is cold-welded to the substrate 54 or amaterial over the substrate (such as an optional metal strike layer 53),thereby transferring the patterned metal 52 from the stamp 51 to thesubstrate 54.

[0059] FIGS. 6(a), (b) and (c) show a master for a PDMS stamp, and thePDMS stamp which was fabricated from the master. Specifically, FIG. 6(a)shows a top view of a master. From the master shown in FIG. 6(a), a PDMSstamp is fabricated using a molding process. FIGS. 6(b) and 6(c) showSEM images of the PDMS stamp fabricated from the master shown in FIG.6(a).

[0060] A second advantage of embodiments of the present inventionresults from the fact that the optimum pressure for the subtractiveprocess increases with metal thickness, ultimately determining themaximum thickness of the patterned metal film practically achievable.However, for the present metal additive process, the optimum pressure isbelieved to be relatively thickness-independent.

[0061] A third advantage is that stamps of embodiments of the presentinvention are more readily reusable than stamps used in a subtractiveprocess. For the subtractive process, the metal film lifted off from thesubstrate remains on the stamp, often resulting in the cleaning orremoval of the metal film from the stamp after each pressing. On theother hand, in the case of the additive process, the metal layer can bere-deposited on the stamp without pre-cleaning, provided that theadhesion-reduction layer remains on the stamp, and that the thickness ofaccumulated metal layers in regions between the contact areas aresmaller than the height of the pattern ridge. To this end, the patternridge is preferably of a sufficient height in relation to the thicknessof the accumulated metal layers in regions between the contact areas.For example, with a pattern ridge height of about 100 μm and a depositedmetal thickness on the stamp of about 0.1 μm, after about 1000 (100μm/0.1 μm) metal depositions and stamp pressings, the regions betweenthe contact areas on the stamp will be approximately filled up with theaccumulated metal layers.

[0062] According to a further embodiment of the invention, no strikelayer is needed in the method for the metal layer to transfer from thestamp to the substrate, and the metal layer may be transferred from thestamp to the substrate by bonding methods known to those skilled in theart other than cold welding. For example, the metal layer may betransferred directly to the substrate, or the metal layer may betransferred to an organic layer, or other material, which is coated onthe substrate.

[0063] An embodiment of the present invention is further directed tometal and/or organic layers patterned over a substrate by the methodsdescribed herein, and to devices formed utilizing the methods describedherein including, for example, OLEDs and arrays of OLEDs.

[0064] Organic materials, including small molecule organic materials,are optionally deposited over the substrate depending on the devicebeing formed and the desired use of the patterned metal. For instance,if the patterned metal is to be used as an anode or a cathode in anorganic light emitting device, organic materials will likely bedeposited over the substrate at some point, either before or after ametal and/or organic layer is patterned over a substrate by the methodsdescribed herein.

[0065] The metal layer over the stamp may include two or more metallayers, so long as the layer furthest from the stamp is capable ofadhering to the substrate or material thereover, preferably cold-weldingto a strike layer, such as a thin metal film. Any additional metallayers over the stamp that do not come into contact with the substrateor material thereover need not be materials that are capable ofcold-welding or otherwise adhering to the underlying material.Accordingly, metals such as chromium and aluminum may be used as one oftwo or more metal layers that make up the metal layer over a stamp, eventhough such metals may not be ideal candidates for cold welding.

[0066] Embodiments including at least two layers of metal over the stampmay be preferred for example, when the patterned metal layer over thestamp is being used as an etching mask, depending on the selectivity ofan etching process. The additional metal layers may be advantageous whenthe patterned metal layer(s) is transferred to a substrate by themethods described herein, and layers under the patterned metal layer(s)are being etched, to prevent all of the patterned metal layer from beingetched away prior to the completion of desired etching of any layersunder the patterned metal layer(s). The etch rate depends on thematerial being etched and the process by which it is being etched.Therefore, it may be desirable to have as a second metal layer over astamp (which is transferred to the substrate by embodiments of themethod of the invention), a metal that has a slower etch rate than thefirst metal layer and/or the material that will be etched using themetal(s) as a mask.

[0067] In a further embodiment of the invention, after depositing afirst metal layer over the stamp, additional organic and/or metal layersmay be deposited onto the stamp over the first metal layer, depending onthe particular device being fabricated, and all of these layers, or onlysome of these layers, may subsequently be transferred to the substrate.For example, one or more organic layers may be deposited over the firstmetal layer, and a second metal layer may be deposited over the one ormore organic layers, and then the second metal layer, the one or moreorganic layers, and the first metal layer may then all be transferredfrom the stamp onto the substrate, or onto the optional strike layer orwhatever material is over the substrate. In addition, one or moreorganic layers may be deposited over the first metal layer, and then theone or more organic layers and the first metal layer may then betransferred from the stamp onto the substrate, or onto the optionalstrike layer or whatever material is over the substrate. Furthermore,one or more organic layers may be deposited over the first metal layer,and then only the one or more organic layers (and not the first metallayer) may then be transferred from the stamp onto the substrate, oronto the optional strike layer or whatever material is over thesubstrate. Such embodiments of the invention could be used to fabricate,for example, an OLED.

[0068] The strike layer, such as for example, a thin metal layer, isdeposited over the substrate by methods known in the art depending uponthe material employed as the strike layer. For example, thermalevaporation is a form of deposition that may be suitable for depositinga thin layer of gold over the substrate. The preferred thickness of thestrike layer varies depending upon, inter alia, the applications of thefabricated device, and the morphology of the layers of the device. Forexample, when a transferred metal is to be used as an electrode in anOLED, the strike layer preferably forms a continuous film, although astrike layer that only forms islands on the substrate (or whatevermaterial is over the substrate) can also be employed. As a continuousfilm, the strike layer allows for a more uniform and consistentfoundation upon which to cold-weld the transferred metal that is to beused as an electrode in an OLED. As a further example, when atransferred metal is to be used as an etch mask, a very thin strikelayer, and even one that only forms islands on the substrate, issufficient. Thus, exemplary strike layer thicknesses include, but arenot limited to, those in the range of from about 5 nm to about 30 nm.

[0069] The metal layer deposited over the stamp is deposited by methodsknown in the art. For example, e-beam evaporation is an example of aform of deposition that may be suitable for depositing gold over thestamp. The preferred thickness of the metal layer deposited over thestamp varies depending upon, inter alia, the applications of thefabricated device, and the composition of the metal layer. Exemplarythicknesses of the metal layer deposited over the stamp include, but arenot limited to, those in the range of from about 30 nm to about 100 nm.In embodiments of the method of the present invention, the stamp and thesubstrate are pressed (or “stamped”) against one another such that theportions of the metal layer over the raised portions of the patternedstamp contact portions of the substrate, or the strike layer over thesubstrate. Sufficient pressure is applied to the stamp and/or thesubstrate, such that the portions of the metal layer that contact thesubstrate, or the strike layer over the substrate, cold-weld thereto.

[0070] When the stamp is applied to the substrate, or the strike layerover the substrate, the substrate may bend such that the device bowsinto depressed portions of the stamp. Contact between the device and thedepressed portion of the stamp is undesirable, and could lead to contactbetween the substrate, or the strike layer over the substrate, andportions of the metal layer that are not on the raised portions of thestamp, and which are supposed to remain on the stamp and not betransferred to the substrate, or the strike layer over the substrate. Toavoid such contact, various parameters may be controlled. For example,stiffer substrates and lower forces applied to the stamp are two factorsthat may be used to eliminate such contact. In addition, if a flexiblesubstrate is used, the substrate may be mounted on a stiff supportstructure, if desired. Still other means may be used to keep theflexible substrate sufficiently rigid to maintain the desiredtolerances. Another important factor is the geometry of the stamp. Inparticular, by increasing the depth of the depressed portions, or bydecreasing the separation between the raised portions, such contact maybe avoided. It is believed that a depth of about 10 microns per 1millimeter of separation is preferred to avoid such contact, althoughthis ratio may change depending upon the particular substrate, stampmaterial and forces.

[0071] The patterned stamp is then removed and the portions of the metallayer that are cold-welded to the substrate, or the strike layer overthe substrate (or otherwise adhered to the substrate or whatevermaterial is over the substrate) break away from the stamp and remaincold-welded (or adhered) to the substrate, or the strike layer (or othermaterial) over the substrate, in substantially the same pattern as thepatterned stamp. To ensure that the cold-welded metal remains over thesubstrate rather than breaking away from the substrate when the stamp isremoved, the relative adhesion between the stamp and the metal layerthereover should preferably be smaller than the adhesion between thestrike layer (or whatever material is over the substrate) and thesubstrate.

[0072] In one embodiment of the present invention, an adhesion-reductionlayer (or adhesion-diminishing layer) is positioned between thepatterned stamp and the metal layer, to lower the adhesion between themetal layer and the stamp. The adhesion-reduction layer may include forexample an organic layer, a TEFLON™ layer or any other material that mayreduce the adhesion between the stamp and the metal layer thereover, bybeing positioned between the stamp and the metal layer. Preferably, theadhesion-reduction layer should reduce the adhesion between the stampand the metal layer thereover a sufficient amount such that the relativeadhesion between the stamp and the metal layer thereover is smaller thanthe adhesion between the strike layer (or whatever material is over thesubstrate) and the substrate. Thus, when the stamp is pulled away fromthe substrate after the metal layer and the strike layer (or whatevermaterial is over the substrate) are cold-welded to one another, thecold-welded metals should remain over the substrate rather than beingpulled off with the stamp.

[0073] The composition and/or thickness of the adhesion-reduction layeris preferably selected to achieve the desired results. Examples ofsuitable adhesion-reduction layers include thin organic layers andTEFLON™. Exemplary thicknesses of the thin organic layer include, butare not limited to, those in the range of from about 25 to about 100 Å.The thin organic layer may optionally be made of Alq₃, which has thefollowing formula:

[0074] The adhesion-reduction layer is deposited over the stamp bymethods known in the art. An example of one suitable method ofdepositing Alq3 for example, is by thermal evaporation.

[0075] In another embodiment of the present invention, at least onelayer is deposited between the substrate and the strike layer. The atleast one layer may be for example, at least one organic layer, whichmay be used for example in forming organic light emitting devices. Inaddition, the at least one layer between the substrate and the strikelayer may include a material suitable in the formation of thin filmtransistors (TFTs), such as CuPc, perylene, pentacene, and othermaterials known in the art.

[0076] According to one embodiment, the at least one layer between thesubstrate and the strike layer includes an adhesion-enhancement layer,which increases adhesion between the substrate and the strike layer.Suitable materials that would perform an adhesion-enhancement functionby increasing the adhesion between the substrate and the strike layerdepend on the materials of the substrate and the materials of the strikelayer, and would be apparent to those skilled in the art. Preferably,the adhesion-enhancement layer should increase the adhesion between thesubstrate and the strike layer thereover a sufficient amount such thatthe relative adhesion between the substrate and the strike layerthereover is greater than the adhesion between the stamp and the metallayer, or whatever material is over the stamp. Thus, when the stamp ispulled away from the substrate after the metal layer and the strikelayer (or whatever material is over the substrate) are cold-welded toone another, the cold-welded metals should remain over the substraterather than being pulled off with the stamp.

[0077] The at least one layer between the substrate and the strike layermay or may not be an organic layer. The at least one layer that isoptionally deposited over the substrate between the substrate and thestrike layer, may be deposited by any suitable method known in the art.For example, when the at least one layer is a polymer layer, the polymerlayer may be deposited for example, using spin coating.

[0078] The thickness of the at least one layer between the substrate andthe strike layer depends on the purpose for which the layer is to beused and the composition of the layer. Suitable thicknesses would beknown to those skilled in the art.

[0079] In another embodiment of the present invention, a first organiclayer(s) may be deposited over the metal layer previously deposited overthe patterned stamp. Additionally, a second organic layer(s) may bedeposited over the substrate, such that the first organic layer(s) andthe metal layer may be transferred from the patterned stamp onto thesecond organic layer(s) over the substrate. This embodiment of theinvention transferring an organic layer from the patterned stamp toanother organic layer could be used, for example, to fabricate an OLED.

[0080] Due to the applied pressure in embodiments of the presentinvention to achieve cold-welding of metals, at least when using a rigidstamp in the methods of the present invention, potential plasticdeformation of the organic layers or any other layers between thesubstrate and the strike layer should be taken into consideration indeciding what materials to use, the thicknesses of the materials and theamount of pressure applied.

[0081] Patterned metal layers over a substrate formed according toembodiments of the method of the present invention may be used forexample, as an electrode or bus line in electrical devices. For example,the patterned metal layer may be used as a cathode layer or an anodelayer in organic light emitting devices (OLEDs) or stacked organic lightemitting devices (SOLEDs) as described for example in U.S. Pat. No.5,707,745.

[0082] After the patterned metal is transferred over a substrate, otherlayers over the substrate may be selectively removed based on thepurpose of the patterned metal layer. For example, the portions of thestrike layer that are not covered by the patterned metal layer, may beremoved by sputtering or other methods known in the art. In addition, inanother embodiment of the invention, the portions of the strike layerthat are not covered by the patterned metal layer are not removed. Forexample, this may be desirable if the fabricated device is to have asubstrate with a metal film thickness variation such that in some areasof the metal film one thickness is needed, and in other areas of themetal film another thickness is needed. Such a variation of the metalfilm thickness could be achieved by not removing the portions of thestrike layer that are not covered by the patterned metal layer.

[0083] In embodiments where an organic layer is positioned between thesubstrate and the strike layer, if desired, after the portions of thestrike layer that are not covered by the patterned metal are removed,the portions of the organic layer that are not covered by the patternedmetal may also be removed by methods known in the art. Portions of theorganic layer may be removed for example, by etching. A non-limitingexample of a suitable form of etching is plasma etching or reactive ionetching (such as anisotropic etching), for example with O₂ or acombination of CF₄ and O₂ to remove the exposed organic layers, i.e.,the parts of the organic layers not covered by the patterned metallayer.

[0084] Similarly, in embodiments where an adhesion-enhancement layer ispositioned between the substrate and the strike layer, if desired, afterportions of the strike layer that are not covered by the patterned metallayer are removed, the portions of the adhesion-enhancement layer thatare not covered by the patterned metal layer may be removed by methodsknown in the art depending on the composition of theadhesion-enhancement layer.

[0085] In embodiments where any other layers are positioned between thesubstrate and the strike layer, if desired, after the portions of thestrike layer that are not covered are removed, the portions of thelayers that are not covered by the patterned metal layer may also beremoved by methods known in the art.

[0086] Additionally, if desired, according to one embodiment of theinvention, portions of all layers over the substrate that are notcovered by the patterned metal layer are selectively removed andsubsequently the portions of the substrate that are not covered by themetal layers are etched to form a patterned substrate. The form ofetching may depend on the composition of the substrate. Suitable formsof etching may include anisotropic etching and other forms of etchingknown in the art.

[0087] According to this embodiment, after the substrate is patternedany portions of layers remaining over the substrate, including thepatterned metal layer, may optionally then be removed from the substrateto yield an uncovered patterned substrate. The removal of the remainingportions of layers may be accomplished by any method known in the art,such as for example, via application of a suitable wet chemical forremoving whatever material is being removed. Preferably, this removaldoes not damage the other portions of layers which are to remain overthe substrate, such as remaining organic layers.

[0088] Embodiments of the method of the present invention in which apatterned metal layer remains over the substrate, may result in apatterned metal layer having a grating line pattern of, for example,about 80 nm wide. In addition, the patterned metal layer can have aresolution of, for example, at least about 100 nm. Regarding the edgesharpness, the patterned metal layer can have a line width of, forexample, about 30 nm.

[0089] One embodiment of the present invention includes a method ofpatterning a substrate, which includes depositing anadhesion-enhancement layer, such as, for example, an organic layer, overa substrate; depositing a strike layer comprising a thin metal film overthe organic layer; pressing a patterned, soft elastomeric stamp havingat least one raised portion and having a metal layer deposited thereoveronto the thin metal film, such that the metal layer over the raisedportion of the patterned, soft elastomeric stamp contacts portions ofthe thin metal film over said substrate, and applying sufficientpressure such that the metal layer and the thin metal film cold weld toone another. According to this embodiment, the patterned, softelastomeric stamp is then removed, and the metal layer is cold-welded toportions of the thin metal film with which it has contact, i.e., itdetaches from the patterned stamp and remains cold-welded to the thinmetal film over the substrate. The metal transferred to the thin metalfilm forms a patterned metal layer over the substrate in substantiallythe same pattern as the at least one raised portion of the softelastomeric stamp. Next, according to the method of this embodiment,portions of the thin metal film that are not covered by the patternedmetal layer are removed, for example by sputtering. Then, portions ofthe organic layer that are not covered by the patterned metal layer areremoved, for example by etching. The portions of the substrate that arenot covered by the patterned metal layer are etched to form a patternedsubstrate. Lastly, according to this embodiment, the patterned metallayer and the remaining portions of the thin metal film and the organiclayer from said patterned substrate are removed to arrive at anuncovered patterned substrate.

[0090] An embodiment of the present invention further relates to devicesformed using the methods of the present invention. Such devices includedevices containing metal layers patterned by the described methods anddevices in which patterned metal layers using the described methods wereused in the fabrication thereof, for example as a mask layer.

[0091]FIG. 7 shows a cross-section of a stamp 1 adapted for use with anembodiment of the present invention and a substrate 4 over which a metallayer is to be patterned. Stamp 1 is preferably formed of a softelastomeric substance such as PDMS. Stamp 1 has raised portions 5, whichmay be formed using techniques known in the art, depending on thecomposition of the stamp, such as silicon patterning and etchingprocesses. The stamp 1 has a metal layer 2 deposited over at least theraised portions 5 of the stamp using techniques known to the art. Inaddition to using stamp 1 from FIG. 7, stamp 1 a as depicted in FIG. 15may be used having raised portions 5 a. The shape of stamp 1 a helpsavoid side wall deposition of a metal when the metal layer 2 isdeposited over the stamp 1 a. Each time “stamp 1” and “raised portions5” are referred to in the description of the figures, it should beunderstood that stamp 1 a and raised portions 5 a may be substitutedtherefor. It is also to be understood that the stamp may be other shapesnot specifically depicted so long as it is patterned so as to be capableof transferring a patterned metal layer to a substrate.

[0092] In the embodiment depicted in FIG. 7, substrate 4 has a thinmetal film 3 (which comprises a strike layer) deposited thereover usingtechniques known in the art. Substrate 4 may be made of any suitablematerial, including glass, polymers, and plexiglass. Substrate 4 may berigid, flexible, opaque or transparent. Preferably, substrate 4 is madeof a substantially transparent material such as glass or plastic. Themetal layer 2 includes a metal that is capable of cold-welding to thethin metal film 3 on compression of the metal layer 2 against the thinmetal film 3. Preferably, the metal layer 2 and the thin metal film 3are non-reactive metals, such as silver and gold.

[0093] Stamp 1 is pressed onto the thin metal film 3, and the portionsof the metal layer 2 over the raised portions 5 of stamp 1 contactportions of the thin metal film 3. Sufficient pressure is applied suchthat the portions of the metal layer 2 that contact portions of the thinmetal film 3 cold-weld thereto.

[0094] The stamp 1 is then removed and the portions of the metal layer 2that are cold welded to the thin metal layer 3, remain cold welded tothe thin metal layer 3 and break away from the stamp, leaving apatterned metal layer 6 (as seen in FIG. 8) over the substrate.

[0095]FIG. 8 shows a cross-section of the stamp 1 and the substrate 4 ofFIG. 7 after portions of the metal from the stamp have been transferredto the substrate to form a patterned metal layer 6 over the thin metalfilm 3, in accordance with embodiments of the methods of the presentinvention.

[0096]FIG. 9(a) shows a cross-section of the substrate 4 of FIG. 8having the patterned metal 6 thereover. Depending on the specific typeof device being made, it may be desirable to remove the portions 7 ofthe thin metal film 3 that are not covered by the patterned metal. Forexample, in order for the substrate to be patterned, the uncoveredportions 7 of thin metal film 3 must be removed before one can get tothe substrate to etch it, for example. The uncovered portions 7 of thethin metal film 3 may be removed for example, by sputtering to yield asubstrate having the patterned metal layer 6 and corresponding portionsof a thin metal film thereover, as depicted in FIG. 9(b).

[0097]FIG. 10(a) shows a cross-section of the substrate of FIG. 9(b)having the patterned metal and corresponding patterned thin metal filmthereover. According to one embodiment of the present invention, ifdesired, the substrate may then be patterned, for example, byanisotropic etching the portions of the substrate 8 that are uncoveredby the patterned metal, to yield the patterned substrate depicted inFIG. 10(b). The pattern of the patterned substrate of FIG. 10(b)essentially corresponds to the pattern of the stamp 1 that was used totransfer the metal over the substrate.

[0098] According to a further embodiment of the present invention, ifdesired, the patterned layers over the patterned substrate as depictedin FIG. 11(a) may then be removed from the substrate to yield anuncovered patterned substrate 9 as depicted in FIG. 11(b). Such layersmay be removed for example by use of a suitable wet chemical dependingon the composition of the layers that are being removed.

[0099]FIG. 12 shows a cross-section of a patterned stamp 1 adapted foruse with an embodiment of the present invention having anadhesion-reduction layer 10, made of for example, a thin organic layeror TEFLON™, between the stamp 1 and the metal layer 2 thereover. Stamp 1is preferably made of a soft, elastomeric material, and has raisedportions 5, which may be formed using techniques known in the art,depending on the composition of the stamp. FIG. 12 also shows across-section of a substrate 4 having a thin metal layer 3 (whichcomprises a strike layer) over the substrate and a layer 11 positionedbetween the substrate and the thin metal layer 3. Layer 11 may includean organic or inorganic material and may be a single layer or aplurality of layers. For example, layer 11 may include the multipleorganic layers of a single or double heterostructure OLED, as describedin U.S. Pat. No. 5,707,745, which is incorporated by reference herein inits entirety. The layer 11 and the thin metal layer 3 are deposited overthe substrate 4 using techniques known in the art. The substrate 4 andstamp 1 may be made of similar materials as those set forth with regardto FIG. 7. Likewise, the thin metal layer 3 and the metal layer 2 mayinclude the metals described with regard to FIG. 7. Stamp 1 is pressedonto the thin metal layer 3, and the portions of the metal layer 2 overthe raised portions 5 of stamp 1 contact portions of the thin metallayer 3. Sufficient pressure is applied such that the portions of themetal layer 2 that contact portions of the thin metal layer 3 cold-weldthereto.

[0100] The stamp 1 is then removed and the portions of the metal layer 2that are cold welded to the thin metal layer 3, remain cold-welded tothe thin metal layer 3 and break away from the stamp, leaving apatterned metal layer 6 over the substrate (as can be seen in FIG. 13).

[0101]FIG. 13 shows a cross-section of the stamp 1 and the substrate 4of FIG. 12 after portions of the metal from the stamp have beentransferred to the substrate 4 in accordance with an embodiment of themethod of the present invention to form a patterned metal layer 6 overthe substrate 4, in accordance with an embodiment of the presentinvention.

[0102] The substrate 4 of FIG. 13 having the patterned metal layer 6thereover can be subsequently patterned employing the same processesdiscussed above and shown in FIGS. 9-11. That is, sputtering,anisotropic etching and wet chemicals, for example, can be employed toremove certain layers and pattern the embodiment as desired.

[0103]FIG. 13(a) shows optical microscope images of an array of200-μm-diameter cathodes for organic light-emitting devices which werefabricated according to an embodiment of the method of the presentinvention. The image shown in FIG. 13(a) was taken after metal transferfrom the stamp and the removal of the strike layer. The uniform patternwas obtained over the whole substrate area. FIG. 13(b) shows a scanningelectron microscope image showing the edge of a cathode, where thepattern resolution is about 1 μm, primarily due to the non-ideal stampshape as shown in FIG. 3.

[0104]FIG. 14 shows the current density (J) vs. voltage (V)characteristics of a 200-μm-diameter organic light-emitting device(OLED) 141 patterned by cold welding (solid circles) according to anembodiment of the method of the present invention. The completed cathodeconsists of 0.5 nm LiF/0.4 nm Al/15 nm Au. Also shown are J-Vcharacteristics for 400-μm-diameter OLEDs 142, 143 patterned byconventional shadow mask methods (open circles (143): a device with abilayer cathode consisting of 0.5 nm LiF/100 nm Al; solid squares (142):an OLED with the cathode identical to that of the stamped device). Theinset to FIG. 14 shows a plot of external quantum efficiency vs. currentdensity for the devices in FIG. 14.

[0105] In an embodiment of the present invention, the stamp should beproperly positioned during the stamping process. In particular, thestamp should be positioned accurately with respect to features alreadyover the substrate 4 during the stamping illustrated, for example, inFIGS. 7, 8 and 12. This alignment may be achieved using techniques knownin the art, such as optical alignment using IR light projected throughthe bottom of the substrate 4, fiducial alignment using lightscattering, and any other suitable technique.

[0106] Although various embodiments of the invention are illustratedwith simplified organic layers and metal layers, additional layers andsublayers may be present, including both organic layers and metallayers. For example, layers between the substrate and the strike layer(e.g., the layer 11 depicted in FIG. 12) may include multiple sublayers,and additional layers may also be present between the substrate and thestrike layer. For example, in an embodiment of the invention, an organiclayer comprising a hole transporting layer, an electron blocking layer,an emissive layer, a hole blocking layer, and an electron transportinglayer can be deposited over the substrate, and a strike layer can bedeposited over the organic layer, such that the metal layer from thestamp is transferred onto the strike layer. For another example, apatterned bottom electrode may be fabricated over the substrate 4 priorto depositing an organic layer, such as for example, the organic layerdescribed in the preceding sentence. Additional organic layers, such asa hole injecting layer, may also be present, such as described in U.S.Pat. No. 5,998,803 to Forrest et al., which is incorporated herein byreference in its entirety. Additional layers as known in the art mayalso be present, such as those described on pages 6-11 of U.S. patentapplication Ser. No. 10/288,785 (filed Nov. 6, 2002), which isincorporated herein by reference in its entirety.

[0107] An embodiment of the present invention includes fabricatingtransparent or top-emitting OLEDs that exploit the high opticaltransmission of compound cathodes, such as Mg:Ag/ITO, in a passivematrix display, but without having such devices limited by the lowerelectrical conductivity of such compound cathodes by the use of buslines deposited by the present methods. Furthermore, an embodiment ofthe present invention provides vapor deposited, electrically conductivematerials without encountering the shorting problems that may beexperienced whenever such electrically conductive materials undergosubstantial scattering during the deposition process.

[0108] Some OLED applications may involve side-emitting OLEDs, in whichcase both electrodes may be non-transparent. Although the electrodes,organic layer and barrier layer are described and illustrated as singlelayers, they may each include various sublayers as known in the art. Forexample, the organic layer may include the sublayers of a single ordouble heterostructure OLED, as described in U.S. Pat. No. 5,707,745,which is incorporated herein by reference in its entirety. In addition,embodiments of devices of the present invention may include additionallayers known in the art that are not illustrated, such as for example, ahole injection enhancement layer or a protective top layer.

[0109] Devices fabricated in accordance with an embodiment of thepresent invention may be incorporated into a wide variety of products.For example, a large, multi-color array of organic light emittingdevices (OLEDs), fabricated using embodiments of the method of thepresent invention to deposit electrical contacts and/or organic layersof the OLED, may form a display, including flat panel displays, bothactive-matrix and passive-matrix displays. Such a display may beincorporated into other products, such as a vehicle, a television, acomputer, a printer, a screen, a sign, a telecommunications device or atelephone, in a manner known to one of skill in the art.

[0110] OLEDs fabricated in accordance with an embodiment of the presentinvention may also be used for applications other than displays. Forexample, a line of such OLEDs could be incorporated into a printer, andused to generate images, in a manner known to one of skill in the art.

[0111] An embodiment of the present invention may also be used tofabricate a wide variety of devices in addition to OLEDs. For example,the present methods may be used to fabricate thin film transistors,photodetectors and other devices in which a high resolution is desired.Embodiments of the present invention may be used to fabricateopto-electronic devices as well, such as arrays of photovoltaic cells orphotodiodes.

[0112] The present invention will now be described in detail withrespect to showing how certain specific representative embodimentsthereof can be made, the materials, apparatus and process steps beingunderstood as examples that are intended to be illustrative only. Inparticular, the invention is not intended to be limited to the methods,materials, conditions, process parameters, apparatus and the likespecifically recited herein.

EXAMPLES Example 1

[0113] A method according to an embodiment of the present invention forthe direct patterning of metal over a substrate by stamping wasperformed. Specifically, a method for high resolution patterning ofmetal cathode contacts for organic electronic devices using low pressurecold welding was performed. In this example, the contacts are formed bytransferring a metal layer from a patterned, soft elastomeric stamp ontounpatterned organic and metal layers predeposited onto a substrate.According to the method of the invention embodied in this example, anarray of efficient electrophosphorescent organic light emitting devices(OLEDs) was fabricated.

[0114] In this example, a method as shown in FIG. 2 was used. FIG. 2describes the cold welding process used to pattern an array ofelectrophosphorescent OLEDs. The substrate 104 was comprised of a (12mm)² glass slide. The substrate 104 was pre-coated with a transparentand conductive layer of indium tin oxide (ITO), which served as theanode of the OLED structure. Organic layers 111 were deposited over theITO layer as follows (as described in M. A. Baldo et al., Appl. Phys.Lett. (1999), 75, 4): about 60-nm-thick hole transporting layer of4,4′-bis[N-(1-napthyl)-N-phenyl-amino] biphenyl (α-NPD); about20-nm-thick light-emitting layer of 4,4′-N,N′-dicarbazole-biphenyl (CBP)doped at 7% by weight with the guest phosphor, factris(2-phenylpyridine) iridium (Ir(ppy)₃); about 10-nm-thick exciton andhole blocking layer of 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline(BCP); and an about 40-nm-thick electron transporting layer of Alq₃. Astrike layer 103 comprising an about 0.5-nm-thick layer of LiF, an about0.4-nm-thick layer of Al, and capped by an about 15-nm-thick layer ofAu, was then deposited over the 0organic layers 111. Prior to thedepositions, the substrate 104 was cleaned by the procedure previouslydescribed in P. E. Burrows et al., J. Appl. Phys. (1996), 79, 7991, andthe organic layers 111 and strike layer 103 for the OLEDs were depositedby high vacuum (˜10⁻⁶ Torr) thermal evaporation. All of the organicmaterials used for OLEDs were purified by thermal gradient sublimationbefore use.

[0115] A patterned, soft, elastomeric stamp 101 comprised an about100-μm-thick layer of PDMS patterned with raised portions 105, and wassupported on a glass slide. A 25-μm-thick photoresist layer (SU-8 50,MicroChem Corporation, Newton, Mass. 02464) on a Si wafer was patternedinto a square lattice of 200-μm diameter cylindrical recesses byconventional photolithography to form the master for the PDMS stamp (seeFIG. 6(a)). PDMS prepolymer (Sylgard 184, Dow Corning Corporation,Midland, Mich. 48686) was poured into the master, and then cured on ahot plate at about 70° C. for about 24 hours while being pressed with a13-mm-thick glass slide. The thickness of the PDMS layer was about 100μm. Pressing was done using a conventional semiconductor flip-chipbonder (M-8HP, Research Devices, Inc., Piscataway, N.J. 08854) (seeFIGS. 6(b) and 6(c)).

[0116] A layer 110 comprising an about 100-nm-thick layer of2,9-dimethyl-4,7-diphenyl-phenanthroline (“bathocuproine” or “BCP”) wasdeposited over the stamp 101, and a metal layer 102 was deposited overthe layer 110. In this embodiment of the invention, the addition of theBCP layer 110 serves to smooth out the metal layer 102 in its adhesionthereto, as compared to if the metal layer 102 was directly adhered toPDMS stamp 101. The metal layer 102 was comprised of an about100-nm-thick layer of Au.

[0117] The stamp 101 was pressed onto the strike layer 103 such that theportions of the metal layer 102 over the raised portions 105 of thestamp 101 contacted portions of the strike layer 103 (see FIG. 2(a)).Sufficient pressure was applied to the stamp 101 such that the portionsof the metal layer 102 over the stamp 101 that contacted portions of thestrike layer 103 cold-welded to the strike layer 103. In this example,the average applied pressure was approximately 180 kPa (corresponding to500 g over the contact area of 0.28 cm²), which is about 1000 timeslower than that previously reported with a rigid stamp (see C. Kim etal., Science (2000), 288, 831; C. Kim et al., Appl. Phys. Lett. (2002),80, 4051).

[0118] As shown in FIG. 2(b), the patterned, soft, elastomeric stamp 101was then removed and the portions of the metal layer 102 from over thestamp 101 that cold-welded to the strike layer 103 remained cold-weldedand broke away from the stamp 101, leaving a patterned gold layer overthe substrate 104.

[0119] Next, as shown in FIGS. 2(b) and 2(c), the uncovered portions 107of the strike layer 103 between the transferred pattern were removed byAr sputter etching in a conventional reactive ion etch system. Arsputter etching was carried out in a parallel-plate (diameter of about24 cm) reactive ion etch system (PlasmaTherm 970 series) for 9 minutesat 20 mTorr and 50 W. The device characteristics were measured underambient conditions. In the devices of this example, it was not necessaryto remove the organic material between the transferred pattern, as thedevices were well isolated due to the high lateral resistivity of theorganics. However, a different gas composition can be used to remove theorganic material as well, if so required.

[0120] Both before and after the patterned gold layer was transferredonto a substrate as set forth above, in accordance with an embodiment ofthe present invention, scanning electron microscope (SEM) images weretaken of the patterned, soft, elastomeric stamp 101. These SEM imagesare shown in FIG. 3. A detailed view (FIG. 3(b)) of the PDMS postreveals that the stamp edge is rounded, resulting in a continuous Aucoating along its top corner. When the stamp was separated from thesubstrate, the Au film was irregularly fractured along this boundary(FIG. 3(c)). The Au pattern was uniformly transferred to the substrateover the whole substrate area with a transfer yield exceeding 97%, andthe pattern edge resolution was approximately 1 μm. The resolution wasprimarily limited by the rounded edges of the stamp posts, leading toirregular edge transfer, as shown in FIG. 13.

[0121]FIG. 14 shows the results of a comparison between the electricaland optical performance of an electrophosphorescent OLED 141 formed bycathode stamping (i.e., in accordance with embodiments of the method ofthe present invention) and that of a control device patterned byconventional shadow mask methods. Two types of control devices wereprepared: one 142 with an identical cathode layer structure to thestamped devices of Example 1 (˜0.5 nm LiF/˜0.4 nm Al/˜15 nm Au), and theother 143 with a bilayer cathode consisting of a 0.5-nm-thick layer ofLiF followed by a 100-nm-thick layer of Al. The diameter of the controldevices was 400 μm, or twice that of the stamped OLEDs of Example 1.

[0122] The measurements in FIG. 14 show that the stamping and strikelayer removal processes do not affect the device performance even thoughpressure is applied directly to the active area of the organicheterostructure during stamping. For example, the voltage correspondingto a current density of J=10 mA/cm² was (9.2±0.3) V for both stamped andcontrol devices, and the external quantum efficiencies (η) at J=1 mA/cm²of the stamped and control devices with the same cathode structure were(6.0±0.3)%. Note that this is about 70% of that for the control devicewith the bilayer cathode. The shapes of η-J curves are nearly identical,indicating that our process does not introduce additional routes leadingto non radiative loss of excitons. The difference in η for devices withthe trilayer and bilayer cathode structures may be due to the differencein reflectivity of Au and Al. We calculated that η for the device withthe Au cap layer is about 83% of that for the device with the Al cap. Inthe calculation, the ultrathin LiF/Al layer was ignored, and lightemitted from an isotropic source was summed with light reflected fromthe cathode. In this case, we employed the complex refractive indices ofcontact materials, as would be understood by one of skill in the art.From these simple considerations, therefore, we conclude that thedifference in the efficiencies between the bilayer and stamped trilayercathodes apparent in FIG. 14 are primarily the result of the differencein reflectivity of Au and Al.

[0123] In Example 1, the pattern size was 200 μm, with an edgeresolution of about 1 μm. H. Schmid et al., Macromolecules (2000), 33,3042, previously reported that PDMS is too soft for patterning featuressmaller than 500 nm. This problem has been overcome by employing “hybridstamps” or “composite stamps,” where the pattern shape is formed in anouter layer of a stiffer polymeric composite such as ‘h-PDMS’, while asofter inner layer (such as PDMS) provides conformability (see H. Schmidet al., Macromolecules (2000), 33, 3042; T. W. Odom et al., Langmuir(2002), 18, 5314). With this approach, this embodiment of the method ofthe present invention will be capable of low pressure patterning ofmetal films with sub-micron features. An additional aspect of lowpressure patterning is its compatibility with conventional semiconductorflip-chip bonders such as that used in the current example. This allowsfor precise (˜1 μm) stamp positioning accuracy, making high resolutionmulti-level stamping of full color displays, for example, achievable bythis simple method.

[0124] Embodiments of the method of the present invention have severaladvantages over previously reported patterning techniques. For example,the present method is very cost-effective, because the stamps arereusable. Preferably, any metal remaining on the stamp is left on thestamp and additional new metal may be added to the stamp as desired orneeded. In addition, if removal of any metal remaining on the stamp isdesired, such metal may be removed by methods known to those skilled inthe art. The metal may be removed for example, by wet etching.

[0125] Embodiments of the method of the present invention are alsoadvantageous over previously reported patterning techniques because thepresent invention offers high throughput. Large areas, such as displaypanels, can be patterned in one step.

[0126] Furthermore, unlike other patterning processes based on materialtransfer, the embodiments of the method of the present invention arecapable of metal patterning without the use of wet chemical or hightemperature processes. In addition, since the metal-organic interface isformed by thermal evaporation similar to that used in conventionalshadow masking, efficient charge injection into organic materials is notaffected by the process, as shown in FIG. 14 herein. This property,combined with the low pressure patterning capability, make this methodsuitable for roll-to-roll fabrication processes of a wide range oforganic electronic devices including OLEDs, organic thin-filmtransistors, and photovoltaic cells. By using roller stamps, large areapatterning can be performed more easily for flexible substrates, sinceoptimum pressure can be applied with smaller forces due to decreasedcontact areas. Embodiments of the method of the present invention allowsimple, cost-effective and high throughput fabrication of OLEDs andother electronic devices and can be applied to the fabrication of flatpanel displays, for example.

[0127] While the present invention is described with respect toparticular examples and preferred embodiments, it is understood that thepresent invention is not limited to these examples and embodiments. Inparticular, the present invention is not limited to OLEDs, or thin filmtransistors and may be applied to a wide variety of electronic devices.In particular, embodiments of the method of the present invention may beused in forming any device in which a patterned metal or organic layeris used in the device itself or in the formation of the device, forexample as an etching mask in patterning other layers or the substrate.The present invention is not limited to the particular examples andembodiments described. The present invention as claimed thereforeincludes variations from the particular examples and preferredembodiments described herein, as will be apparent to one of skill in theart.

What is claimed is:
 1. A method of fabricating a device, comprising:depositing a metal layer over a patterned, soft elastomeric stamp; andtransferring the metal layer from the patterned, soft elastomeric stamponto a substrate.
 2. The method of claim 1, further comprising: prior todepositing the metal layer, depositing an adhesion-reduction layer overthe patterned, soft elastomeric stamp, such that the metal layer isdeposited over the adhesion-reduction layer.
 3. The method of claim 1,further comprising: prior to transferring the metal layer onto thesubstrate, depositing a first organic layer over the metal layer overthe patterned, soft elastomeric stamp; prior to transferring the metallayer onto the substrate, depositing a second organic layer over thesubstrate; and during the transfer of the metal layer to the substrate,transferring the first organic layer and the metal layer from thepatterned, soft elastomeric stamp onto the second organic layer over thesubstrate.
 4. The method of claim 1, further comprising: prior totransferring the metal layer onto the substrate, depositing an organiclayer over the substrate and a strike layer over the organic layer, suchthat the metal layer from the patterned, soft elastomeric stamp istransferred onto the strike layer such that the metal layer from thepatterned, soft elastomeric stamp is in direct contact with the strikelayer during the transfer.
 5. The method of claim 4, further comprising:prior to depositing the metal layer, depositing an adhesion-reductionlayer over the patterned, soft elastomeric stamp, such that the metallayer is deposited over the adhesion-reduction layer.
 6. The method ofclaim 4, wherein the strike layer comprises a metal.
 7. The method ofclaim 4, wherein the strike layer comprises an organic material.
 8. Themethod of claim 4, wherein the strike layer and the organic layer are ingood electrical contact.
 9. The method of claim 6, wherein depositingthe strike layer further comprises depositing a layer of Al and LiF,followed by depositing a layer of Au.
 10. The method of claim 9, whereinthe layer of Al and LiF is less than about 1 nm thick, and the layer ofAu is less than about 15 nm thick.
 11. The method of claim 6, whereinthe strike layer is about 5 to about 16 nm thick.
 12. The method ofclaim 4, wherein the metal layer comprises Au.
 13. The method of claim6, wherein the method is performed in a vacuum.
 14. The method of claim6, wherein the metal layer from the patterned, soft elastomeric stamp istransferred onto the strike layer without the formation of an oxidelayer.
 15. The method of claim 6, wherein the metal layer is about 30 toabout 100 nm thick.
 16. The method of claim 4, further comprising:removing portions of the strike layer that are not covered by thetransferred metal layer.
 17. The method of claim 16, wherein theportions of the strike layer are removed by sputtering.
 18. The methodof claim 4, wherein the metal layer is transferred from the patterned,soft elastomeric stamp onto the strike layer by applying a pressure ofabout 180 kPa or less.
 19. The method of claim 4, wherein the metallayer from the patterned, soft elastomeric stamp is transferred onto thestrike layer with a resolution of greater than or equal to about 100 nm.20. The method of claim 4, wherein the stamp comprisespoly(dimethylsiloxane).
 21. The method of claim 4, wherein the stampcomprises an outer layer of a stiff material attached to a soft innerlayer.
 22. The method of claim 21, wherein the outer layer comprisesh-PDMS and the inner layer comprises PDMS.
 23. The method of claim 4,wherein the organic layer comprises a small molecule organic material.24. The method of claim 4, wherein the organic layer further comprises ahole transporting layer, an electron blocking layer, an emissive layer,a hole blocking layer, and an electron transporting layer, deposited inthat order over the substrate.
 25. The method of claim 4, wherein theorganic layer is patterned into regions capable of emitting differentspectra of light.
 26. The method of claim 4, further comprising:fabricating a patterned bottom electrode over the substrate prior todepositing the organic layer.
 27. A method of fabricating a device,comprising: depositing a first metal layer over a patterned, softelastomeric stamp; depositing a first organic layer over the first metallayer; depositing a second metal layer over the first organic layer; andtransferring the second metal layer, the first organic layer and thefirst metal layer from the patterned, soft elastomeric stamp onto asubstrate.
 28. The method of claim 27, further comprising: prior todepositing the first metal layer, depositing an adhesion-reduction layerover the patterned, soft elastomeric stamp, such that the first metallayer is deposited over the adhesion-reduction layer.
 29. A method offabricating an organic device, comprising: depositing a first organiclayer onto a patterned, soft elastomeric stamp; and transferring thefirst organic layer from the patterned, soft elastomeric stamp onto asubstrate.
 30. The method of claim 29, wherein the first organic layercomprises an active component of the device.
 31. The method of claim 29,further comprising: prior to depositing the first organic layer,depositing an adhesion-reduction layer over the patterned, softelastomeric stamp, such that the first organic layer is deposited overthe adhesion-reduction layer.
 32. The method of claim 29, furthercomprising: prior to depositing the first organic layer, depositing ametal layer over the patterned, soft elastomeric stamp, such that thefirst organic layer is deposited over the metal layer.
 33. The method ofclaim 29, further comprising: prior to transferring the first organiclayer onto the substrate, depositing a second organic layer over thesubstrate, such that the first organic layer from the patterned, softelastomeric stamp is transferred onto the second organic layer such thatthe first organic layer is in direct contact with the second organiclayer during the transfer.
 34. The method of claim 33, wherein the firstorganic layer is transferred from the patterned, soft elastomeric stamponto the second organic layer by applying a pressure of about 180 kPa orless.
 35. The method of claim 4, wherein the strike layer comprises acontinuous film.